Pulmonary Diffusion Explained! Causes of Hypoxemia #2 of 5
09
October

By Adem Lewis / in , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , , /


welcome to another MedCram lecture
we’re gonna talk about hypoxemia again and this time we’re gonna talk about
diffusion again hypoxemia literally meaning low oxygen in the blood let’s
talk about diffusion as a possibility for a mechanism for hypoxemia to talk
about that though we’ve got to sort of break down the the alveolus and i’m
drawing that here and then we’ll switch to a blue color this is to demonstrate
the pulmonary artery which as you know is deoxygenated and we have the
capillary and then we’ll switch to red to show that in fact the blood is now
oxygenated and then this green is gonna be oxygen because you know that oxygen
tanks are always green right okay so we’ve got oxygen coming down into this
area and then there’s something that we should talk about called the capital a
lowercase a gradient okay so you guys know how a lung works john west is
quoted as saying the lungs are very very easy to understand air goes in and out
and blood goes round and round well let’s sort of break that down a
little bit to understand it a little bit better oxygen is going to come in into
the alveolus and it’s going to diffuse past this one cell layer thick
epithelium which is the epithelium of the lung and then it’s going to go
through this one cell layer thick endothelium which is could then go into
the pulmonary artery pulmonary capillary and then pulmonary vein which is going
to become oxygenated the problem here is that sometimes you can have a blockage
in this area and that can cause a diffusion abnormality where you’d see
this is particularly in patients with infiltrated diseases of the lung like
pulmonary fibrosis especially if they exercise let me give you an example if
you’ve got a patient with fibrosis and they’re at rest so that means there’s
stuff in the middle here in this interstitial and it’s preventing oxygen
from diffusing in these red blood cells normally to get through take about zero
point seven five seconds to transmit through this pulmonary capillary and to
pop out the other side with oxygen now as they’re going that slowly they’re
able to equilibrate with the oxygen here in the alveolus that’s no problem
that means the oxygen coming out the other end is you know 95 percent
saturated however when they start to exercise cardiac output goes up and even
though there are parallel systems that the blood can go through to pick it up
and the velocity of these red blood cells increase and so therefore the 0.75
seconds goes down and so it’s going through its equilibrate and it pops out
when they’re exercising before there’s full equilibration so now maybe instead
of 95 percent saturated it’s only 85 percent saturated let’s just say and so
what you get there is hypoxemia because the red blood cells sped up but this
happened primarily because there was some sort of a deposition interstitial
deposition in here that prevented this oxygen from diffusing into the pulmonary
capillary and there you’ll see an increased a a gradient capital a
standing for alveolus lowercase a standing for artery so that increased a
gradient you’ll see and we’ll show you how to calculate that in a bit
here’s a equation for the AAA gradient and this kind of has some of the items
in it that we saw before this part up here refers to the capital a that’s the
amount of oxygen in the alveolus this here is the smaller case a this is the
amount of oxygen that is dissolved in the blood this is the pao2 that you pick
up on a blood gas so let’s plug in some numbers for somebody whose normal fio2
in this case would be point 2 1 p.m. is fair let’s say this is at sea level that
would be 760 millimeters the P of water is about 47 and the pco2
you would be 40 divided by 0.8 okay so let’s go through and oh let’s say po2
what’s the normal po2 around 86 so 760 minus 47 is 713 this times point to one
is gonna give us 150 150 minus 50 is a hundred a hundred minus 86 is equal to
about 14 so the a a gradient is about fourteen which is pretty normal in terms
of trying to figure out or guesstimate what a normal a a gradient would be it
should be around twenty fifteen twenty or so another way of trying to predict
it some people say is if you use this type of an equation where you take the
age of the patients and divide it by four and then you just add four so a
40-year old gentleman 40 divided by four is ten plus four is fourteen so that’s
how you calculate a an a a gradient now notice if for some reason we ran into a
situation where there was a diffusion problem what would we see we would have
to see that the fio2 would go up because we’d have to give them more oxygen to
keep the oxygen the way it was correct this would not change this would not
change and this would either stay the same or go down we’d have to give a lot
of this to keep this the same you can see that in situations where we can’t
get oxygen through we’ve got to go up on the capital a to get the same or only a
small small increase in the little a and so what I’m saying here is that a large
a a gradient means there’s some barrier or some problem in oxygenating the
tissue or getting oxygen into the pulmonary capillary okay then so let’s
review if we look at diffusion couple key points that you need to know
first of all it does respond to a hundred percent oxygen again if we look
at our alveolus and we’ve got our pulmonary artery and our pulmonary vein
if there’s a barrier hair just giving more oxygen will help push it through
and get the o2 in the pulmonary vein to come up so it does respond to 100%
oxygen it has an increased AAA gradient where do we see this fibrosis any kind
of situation where there is a collagen deposition here in the interstitial of
the lung like component fibrosis especially if they’re exercising and why
is that we’re going to emphasize that with exercising because the red blood
cells are going through here at a faster velocity and therefore it accentuates
this difficulty in diffusion but you could have to have a lot of thickening
for that to happen okay so that takes care of diffusion we’re going to move on
to other ones in the next lecture


4 thoughts on “Pulmonary Diffusion Explained! Causes of Hypoxemia #2 of 5

  1. See short videos on the 4 other main causes of hypoxemia free at our website: https://www.medcram.com/courses/hypoxemia-explained-clearly

  2. Great video MedCram. I can measure this diffusion capacity for the lung in clinical practice in the respiratory lab. This is very helpful in diagnosing disease causing hypoxemia and breathlessness. I’ll demonstrate this in a video soon as it complements your physiological explanation

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